The invention relates to a hybrid vehicle powertrain that includes an engine and a hydraulic or pneumatic drive having limited energy capacity compared to that of the engine. More particularly, the invention pertains to controlling torque supplied to the wheels from multiple power sources.
Hydraulic Power Assist (HPA) is a type of hydraulic hybrid vehicle, in which energy from regenerative braking or from an engine is stored in a hydro-pneumatic accumulator, and the conversion between mechanical power and hydraulic power is achieved through high pressure pump/motor having a variable volumetric displacement. In an HPA system, using stored energy from regenerative braking to help accelerate the vehicle reduces the burden on the engine and reduces fuel use.
Because of the high power density available with such hydraulic systems, it is possible to recover efficiently a significant portion of braking energy with an HPA system comprised of a single pump/motor and storage accumulators. With a 7000 lb. vehicle and a pump/motor whose maximum displacement is 150 cc., an HPA system can recover 72 percent of the available braking energy in the Environmental Protection Agency (EPA) city cycle. The pump/motor operates for long periods at higher displacements and with a relatively high cycle average efficiency of 88 percent. With a return of 56 percent of the braking energy to the drive wheels (72 percent recovered in braking, and 88 percent transfer efficiency in both pumping and motoring), it is possible to recover 56 percent of the vehicle kinetic energy (or 75 percent of the velocity) while accelerating, neglecting road load friction. In the EPA city cycle, it was possible to fill the hydraulic system when braking from 30 mph and then moderately accelerate again to about 22 mph using only stored energy from the HPA system.
U.S. Pat. No. 5,495,912 describes a hybrid powertrain in which engine output speed is controlled for optimum efficiency by adjustment of the input speed of a continuously variable transmission (CVT). Where power more than that provided by the engine is required, additional power is input to the drivetrain from a fluid motor driven by fluid pressure stored in an accumulator. When the engine produces power greater than that demanded by the vehicle, the fluid motor is reversed for operation as a pump and excess engine power is used to drive the pump and store energy in the accumulator in the form of fluid pressure. A controller determines power output required of the engine as a sum of that indicated by a sensor which senses power demanded of the vehicle by a driver, and an increment of power required to maintain the pressure of the accumulator above a threshold amount. An engine speed controller controls the rotary speed of the engine output, to produce the required total power output, by changing the input speed of the CVT. In order to maintain optimum engine efficiency, memory containing maps correlating values for optimum engine speed with values for engine output power, are used to determine the optimum engine speed from the map by reference to the determined total requirement for engine output power. The invention also includes a method of operating the system in order to achieve the objective of optimum engine efficiency.
U.S. Pat. No. 5,505,527 describes a vehicle powertrain having regenerative braking that includes wheels and a brake pedal which, upon engagement, is activated first into a first zone of operation and then into a second zone of operation. A braking detector detects either a released state or an engaged state for the brake pedal and, if in the engaged state, detects if the pedal is in the first or the second zone of operation. Friction brakes brake a pair of the wheels, responsive to detection of the brake pedal within the second zone of operation. The hydraulic portion of the drivetrain includes an accumulator for storing hydraulic fluid under pressure, and a reservoir for storing the hydraulic fluid at a lower pressure. A pump/motor, located in the high-pressure line, operates as a motor to drive the drive wheels in a drive mode and as a pump driven by the drive wheels in a braking mode. A prime mover, having its inlet connected to the reservoir through a low-pressure line and its outlet connected to the accumulator through a high-pressure line, hydraulically drives the pump/motor in its motor mode. A controller switches the pump/motor into the braking mode in responsive to detection of an engaged state for the brake pedal and into the drive mode in responsive to detection of the released state of the brake pedal. A switch valve connects the high-pressure line to the accumulator in the braking mode and to the reservoir in the drive mode.
A principal purpose of a hybrid powertrain having two power sources, an internal combustion engine and high-pressure source of pneumatic or hydraulic fluid, is an increase in fuel economy compared to conventional powertrains having only an engine. The kinetic energy of the vehicle provides a replenishable source of energy that can be readily recovered while braking the vehicle, efficiently converted to potential energy, and stored as high-pressure fluid for use in accelerating the vehicle. The engine also can be used to pump fluid to a high-pressure accumulator from a low-pressure reservoir when engine output power is available for this purpose and fuel economy permits.
There is a need, however, for a method to control torque supplied from different power sources when accelerating a vehicle, particularly where one power source has a limited energy capacity compared to that of the engine, but has substantial potential to improve fuel economy while accelerating the vehicle. The method and system of this invention allow the seamless integration of a torque source that can provide vehicle acceleration in addition to the internal combustion engine.